One of the problems with making high speed measurements with a logic analyzer probe or a mixed signal oscilloscope probe is the probe interface that perturbs the system under test (DUT). Generally, there are two types of probe interfaces, a connectorless probe interface and a connector probe interface. The connectorless probe interface has electrical contacts in a probe head that mate with corresponding electrical contacts mounted on the DUT, such as described in U.S. Pat. Nos. 6,888,361 and 6,781,391, which are incorporated herein by reference. Connector type probe interfaces have a receptacle portion that is mounted on the DUT. The receptacle portion has electrical contacts that mate with corresponding electrical contacts on the DUT. A plug portion having electrical contacts corresponding to the electrical contacts of the receptacle portion is mounted on the probe head and mates with the receptacle portion.
Both types of interfaces have unterminated stubs that reflect energy back into the device under test. Electrical stubs can impact the circuit being probed, so probe designers seek to minimize the stubs. However, existing probing methods place input resistors approximately 0.100 inches from the DUT contact points of the circuit to be probed, creating a ‘stub’ on the probed circuit. Two elements contribute to the ‘stub length’: (1) the length of the electrical contacts connected to the electrical contacts on the DUT, and (2) limitations of placing resistors close to the edge of the circuit board of the probe head. The length of the time delay in the compliant contact is one of the factors that limit the speed at which the probe can be used.
In previous implementations of logic analyzer probes, the input resistors have resided on a circuit board in the probe head, and the compliant electrical contacts come into contact with the edge of the circuit board. The length of the contacts and the amount the resistors are set back from the edge of the circuit board both contribute to the unterminated stub length. The distance that the resistors are set back from the edge of the circuit board is determined by circuit board manufacturing constraints. In addition, the edge wrap on the circuit board produces an edge that is somewhat rough and departs from planarity because of the process of routing and plating the laminated circuit board edge.
One connector type probe interface is called a MICTOR® connector, manufactured and sold by Tyco Electronics, Corp., Harrisburg, Pa. A MICTOR connector has a plug and closely mating receptacle. The electrically conductive lines in the plug and receptacle are contained in mating housings. The ends of the electrically conductive lines extending from the bottom of the receptacle are bent at an angle to form contact pads for soldering to parallel rows of conductive pads on the surface of the DUT. The ends of the electrically conductive lines at the other end of the receptacle form electrical contacts that mate with corresponding electrical contacts in the plug when the closely mating plug and receptacle are connected together. Electrical elements, such as resistors, may be mounted in the logic analyzer probe head between the MICTOR plug and additional circuitry in the probe head to provide electrical isolation for the device under test.
Another example of a connector type probe interface is a Spirit® Compression Test Probe, manufactured and sold by Samtec, Inc., New Albany, Ind. The Spirit® Compression Test Probe has mounting brackets positioned on either side of parallel rows of contact points on a circuit board of the DUT. Parallel rows of passive resistors corresponding to the parallel rows of contact points on the circuit board are embedded in a connector housing that is positioned between the mounting brackets. Each passive resistor is secured to a lower spring contact and an upper spring contact with the lower spring contact engaging one of the contact points on the circuit board and the upper spring contact engaging a corresponding contact point in a probe head. The stub length of the lower spring contact is 0.100 inches.